A. Field of the Invention
This apparatus relates to a method and apparatus for measuring the amount of a substance containing atoms of a non-radioactive element of atomic number of 11 or greater on a fabric using X-ray transmission, particularly where the fabric is a carpet and the coating is a calcium-and/or aluminum-containing latex coating.
B. Description of the Prior Art
In the manufacture of conventional carpets, a styrene-butadiene based latex is used to adhere the various components of the carpet together. A typical latex formulation comprises, by weight, about 55 percent calcium carbonate, about 15 percent a styrene-butadiene-itaconic acid terpolymer, and about 30 percent water. In other latex formulations, all or a part of the calcium carbonate may be replaced by alumina trihydrate. In a representative manufacturing process, the yarn, which eventually forms the carpet nap, is tufted into a jute or polypropylene primary backing, the latex is coated on the back of the primary backing, and a second jute or polypropylene backing is then applied. Finally, the carpet is dried. The latex layer serves as a binder which holds the yarn tufts and the second backing in place. Immediately after application of the latex, the partially completed carpet may contain from about 5-80 ounces per square yard (15-270 mg/cm.sup.2) of fabric and about 20-60 ounces per square yard (70-200 mg/cm.sup.2) of wet latex.
Since carpet quality is critically dependent on the latex binder, a precise, reliable instrument for non-destructibly monitoring the amount of latex applied would be quite useful to the carpet manufacturer. Moreover, a portable instrument, readily adapted for interstate transportation in, for example, an automobile, would be a valuable asset for technical assistance representatives of latex vendors.
The term "amount" of latex is used herein because the technical term of art employed in the carpet industry is somewhat of a misnomer: it is common to express the average "thickness" of the latex coating, not in linear units, but rather in terms of weight of latex per unit area of the carpet, e.g. ounces per yard or mg/cm.sup.2. Since the specific gravity of the latex can readily be determined, however, it is a simple matter to convert from units of mass per unit area to linear units, and vice versa.
It is reported by J. F. Cameron et al. in Radioisotope Instruments, Part I, page 153, Pergamon Press, New York, 1971, that beta-gauges before and after the "kiss roll" used to apply the latex make it possible to control the amount of latex applied. Beta-gauges, however, whether based on scattering or transmission phenomena, present rather serious radiation hazards. Moreover, the results are significantly affected by the carpet matrix, i.e., by the components of the carpet other than the latex formulation. Consequently, the accuracy and precision of beta gauges is marginal. For these reasons, beta gauges are not normally used in the carpet industry. For a lack of a better instrument, the instrument believed most widely used at present for measuring the amount of latex applied to a carpet, is based on microwave absorption. Microwave absorption is a function of water content, water being one of the components in a latex formulation. Even this instrument and method, however, suffer from several sources of error and are not well accepted by the industry.
Other approaches to the problem have been considered but found wanting in at least one respect. X-ray fluorescence, for example, was disclosed by Nelson et al. in Textile Research Journal, pp. 357-361 (June, 1973) as a method for determining the amount of a bromine compound on a fabric, but fluorescence is not universally suitable for coatings of the thicknesses encountered in carpet applications. X-ray scattering is much affected by the matrix, and the results obtained would suffer unacceptably from a lack of precision and accuracy.
Radiation transmission, and in particular X-ray transmission, has been used in other applications for determining the thickness of objects, and even for determining the thickness of various coatings on objects.
Friedman, U.S. Pat. No. 2,462,088 discusses early methods, based on X-ray transmission or back radiation, for measuring the thickness of heavy steel sections and pipe walls, and then discloses a method for accomplishing the same objective wherein the beam of radiation is directed perpendicular to the radius of curvature of the pipe and in a plane perpendicular to the longitudinal axis of the pipe, and wherein the thickness of the pipe wall, which may be of several layers, is measured by determining sharp variations in the amount of X-rays transmitted to a detector as a function of the position of the radiation beam along the radius of curvature of the pipe. Wolf, U.S. Pat. No. 2,486,902; Herzog, U.S. Pat. No. 2,540,261; McKee, U.S. Pat. No. 2,702,864; and O'Shea et al., U.S. Pat. No. 3,426,196 also disclose use of radiation to measure the thickness of pipes or other curved walls.
Herzog, in U.S. Pat. Nos. 2,501,173 and 2,528,724, discloses a device and method for measuring the thickness of curved objects such as hollow propeller blades using a homogeneous source of gamma radiation and a detector therefor mounted on a pantograph frame in a manner such that the respective housings for the source and detector can be maintained against the respective internal and external surfaces of the object being measured. Herzog also teaches measurement of the density of a fluid in a container using radiation transmission in U.S. Pat. No. 2,501,174.
Apparatus and/or methods for measuring the thickness of moving metal strips by radiation transmission are taught by McNamara, U.S. Pat. No. 3,179,800; and Busch et al., U.S. Pat. No. 3,715,592.
Apparatus and/or methods for determining coating thicknesses on material other than carpeting using radiation transmission are disclosed by Zemany, 30 Rev. Sci, Instr. 292-3 (1959) -- titanium films on Kovar alloy; and Murray, 4 Advan. X-ray Anal. 309-18 (1961) -- Kaolin coating clay on paper.
Apparatus and/or methods using radiation for determining the density, thickness, or concentration of other miscellaneous materials are disclosed by: Rhodes, U.S. Pat. No. 3,270,200 -- concentration of an element in a mixture; Duftschmidt et al., U.S. Pat. No. 3,497,693 -- discontinuous materials such as asbestos-cement sheets, plastic sheets, or glass sheets; Roller et al., U.S. 3,796,874 -- cable insulation; Dragonette, U.S. Pat. No. 3,531,827 -- plastic sheets; Gladstone, Paper-Maker, Wiggins, Teape & Co. Ltd., London 1956, No. 4-7, abstracted at 50 Chem. Abstracts 13442b; Barta et al., 14 Energia Atomtech. 570-575 (1961), abstracted at 58 Chem. Abstracts 209f -- wall thickness of oxygen cylinders, Joffe et al., U.S. Pat. No. 3,319,067; and Australian Pat. No. 263,677.
Initial attempts at employing X-ray transmission for measurement of the amount of latex applied in the manufacture of carpeting proved commercially unacceptable. In particular, the accuracy and precision obtained was not satisfactory when filtered radiation from a linear array of plutonium-238 was used as a radiation source, in combination with a calcium fluoride scintillation detector coupled to a light pipe and a photo multiplier tube as the means for detecting the transmitted radiation. Even when filtered, several different X-rays of similar energies were nevertheless emitted from such a source, and, since the lower energy X-rays were more readily absorbed than those of higher energies, the percent of radiation transmitted was not a truly exponential function of thickness.